The St. Louis area is bracing for more racial unrest tied to the police killing of unarmed teenager Michael Brown and anger over another police killing of a black teenager Wednesday night is expected to add fuel to the fire.

Several civil rights organizations and protest groups, including one calling itself Hands Up United, plan to mark the weekend with marches and rallies in St. Louis and the suburb of Ferguson, Missouri, where Brown was killed two months ago Thursday.

The groups are trying to force the arrest of the officer who killed Brown, and to draw attention to broad concerns about police treatment of black Americans.

Protest organizers say they are planning only peaceful activities, but police fear Wednesday's killing of the black teen in the south St. Louis neighborhood of Shaw might trigger violent outbursts.

"We never advocate violence or any kind of looting or anything else. But I do know that people were angry last night and they will be out this weekend," said Tory Russell, a leader of Hands Up United. "I don't know what they are going to do."

At least 6,000 people have registered on an organizing website for the "weekend of resistance" events in and around Ferguson, which kicks off Friday with a "justice now" march and will be capped with actions of "civil disobedience" on Monday.

The Hands Up United webpage shows posts from people looking to share rides to St. Louis from Los Angeles, Milwaukee, Dallas, Boston and New York.

Ferguson Mayor James Knowles said law enforcement officers throughout the area are planning for large crowds and possible violence.

"There are a lot of people coming into town," said Knowles.

"We are going to be prepared. There is intel out there that there are people wanting to be bad things. And people who want to cause a problem are going to use that [the St. Louis shooting Wednesday] as a rallying cry," he said.

In the Shaw incident, a white St. Louis police officer fatally shot a black teenager who police said had a gun and was firing at the officer.

The officer was not on duty at the time, but was working for a private security company, according to St. Louis Metropolitan Police Chief Sam Dotson.

The officer, who was not hurt, has been placed on administrative leave and an investigation was under way, police said.

Relatives, who identified the dead teen as 18-year-old Vonderrit Myers Jr., disputed the police version of events and said Myers did not have a gun, the St. Louis Post-Dispatch reported.

The shooting sparked protests that raged until nearly dawn Thursday.

The killing in the Shaw neighborhood comes as the St. Louis area is still struggling with unrest after the Aug. 9 killing of Brown by Ferguson police officer Darren Wilson. Brown was unarmed when shot at least six times, and outrage over his death has fueled weeks of sometimes violent protests.